Anaerobic digestion of biomass occurs in four stages; (1) hydrolysis, (2) acidogenesis, (3) acetogenesis and (4) methanogenesis. During hydrolysis, polymeric organic molecules are broken down to smaller units such as oligomers, dimers and monomers. Depending on starting material, these smaller units are sugars, amino acids or fatty acids. Acidogenesis then converts these molecules to short-chain carboxylic acids, ketones, alcohols, hydrogen and carbon dioxide. During acetogenesis the short-chain carboxylic acids and alcohols are converted to acetic acid, hydrogen and carbon dioxide, which then are converted to methane during methanogenesis. Different stages of anaerobic digestion are performed by distinct groups of microbes. Together these groups form a microbial consortium capable of synergistic digestion of biomass. The consortium consists of (a) hydrolytic microbes and acidogens, (b) acetogens, and (c) methanogens. Biogas produced as the result of anaerobic digestion is a mixture of methane (50-75%) and carbon dioxide (25-50%), as well as minor amounts of other components such as hydrogen sulfide, nitrogen, hydrogen, moisture, oxygen, ammonia and siloxanes.
A significant part of the nitrogen in organic material is present in amino acids which make up the protein fraction of the organic matter of the feedstock material or biomass. In the process of anaerobic digestion, the proteolytic and protein fermenting bacteria are mainly members of the genus Clostridium (Ramsay & Pullammanappallil, 2001). It has previously been reported in co-owned U.S. published patent application US2014/0271438 A1 and co-owned U.S. Pat. No. 8,691,551, that the mixed bacterial population S1 (deposited under the terms of the Budapest Treaty as CBS accession No. 136063) is highly active in degrading nitrogenous compounds in various organic materials through anaerobic hydrolysis and acidogenesis. Simultaneously, the microbial activity releases organic nitrogen as inorganic ammonia/ammonium in a process called ammonification or nitrogen mineralization.
Other sources of nitrogen in organic materials commonly used as feedstock material are urea, uric acid and ammonia present in, e.g. animal urine and manure. In addition, materials such as wastewater sludge can contain a high amount of nitrogen in compounds such as nucleic acids. Plant biomass and silage can be rich in nitrates, among other forms of nitrogen. The feedstock materials typically used in the process of this invention include, but are not limited to, animal by-products, fish by-products, slaughterhouse waste, organic fraction(s) of municipal solid waste, energy crops, food waste, sewage sludge, food industry by-products, and crop culturing by-products.
“Feedstock” or “feedstock material,” as used herein is defined as raw material supplied to a processing plant. Biogas production from nitrogen rich feedstock materials, e.g., organic waste materials, is accompanied by the release of proteinacious nitrogen as ammonia. High ammonia concentrations inhibit the activity of microbes involved in anaerobic digestion, and this in turn leads to an accumulation of short chain carboxylic acids i.e. volatile fatty acids (VFA). A recent study has shown that such high ammonia concentrations cause a decrease in expression of methyl-coenzyme M reductase, the enzyme catalyzing the terminal methane-forming reaction of methanogenesis (Zhang et al. 2014). This leads to a decreased use of acetic acid by acetoclastic methanogens, and a subsequent pH decline caused by the accumulation of VFA. This change in conditions can in turn lead to cessation of protein hydrolysis, acidogenesis and ammonification, as demonstrated in culture with Clostridium sporogenes MD1, where an influx of anionic VFA caused effilux of intracellular glutamate, the universal carrier of amino groups in deamination and transamination reactions of amino acid metabolism (Flythe & Russell 2006). In addition, end product inhibition caused by high ammonia levels is thought to slow down metabolic processes that produce ammonia. Therefore, excess ammonia affects the anaerobic digestion process on many levels, decreasing both the efficiency of its own production as well as biogas production.
Controlling the carbon to nitrogen (C/N) molar ratio of feedstock can reduce the ammonia load during AD. C/N molar ratio expresses the number of carbon atoms present per each nitrogen atom. C/N ratio can also be calculated and expressed as the ratio of masses of carbon and nitrogen. C/N molar ratio can be derived from C/N mass ratio by multiplication by 1.17, i.e. the C/N molar ratio is 17% higher than C/N mass ratio. The calculation is based on the differences in molar mass of carbon and nitrogen atoms. Alternative ways of presenting carbon or nitrogen for C/N ratio calculation include chemical oxygen demand (COD) or total organic carbon (TOC) to represent the amount of carbon and total Kjeldahl nitrogen (TKN) or total nitrogen (signifies total elemental nitrogen or the sum of nitrate NO3−, nitrite NO2−, organic nitrogen and ammonia nitrogen, depending on determination method) to represent the amount of nitrogen.
In biogas production, a high C/N ratio i.e. lack of nitrogen, leads to the inefficient utilization of carbon due to a lowered amount of microbial biomass, whereas a low C/N ratio, i.e. a surplus of nitrogen, can cause ammonia inhibition of methanogenesis. An optimal C/N ratio can be generated through the co-digestion of nitrogen rich and nitrogen poor feedstock materials. However, for example, in the biogasification of nitrogen rich animal slaughter by-products, an increase of COD through co-digestion with carbon rich feedstock did not enhance methane production from the nitrogen rich feedstock (Resch et al. 2011). Two-phase AD for nitrogen rich feedstock materials, with accompanying nitrogen recovery by stripping, has been suggested for lowering ammonia concentration during biogasification (U.S. Pat. No. 6,716,351 B2). The '351 patent process, however, limits its scope to nitrogen rich feedstock materials.
Two-phase AD with separate acidogenic and methanogenic phases has been described previously in the patent literature. For example, U.S. Pat. No. 4,022,665 discloses a two-phase system where the recycling of biogasification reject water back to the first phase, hydrolysis/acidogenesis, is presented as an option. Patent documents U.S. Pat. No. 7,309,435 B2, EP 1,181,252 B1 and EP 2,220,004 B1 describe two-phase systems where control of oxidation-reduction potential, VFA concentration, or pH, respectively, is used to enhance process efficiency. Patent document U.S. Pat. No. 8,642,304 B2 discloses a two-phase system where control of VFA concentration between two methanogenic reactors improves digestion. None of these documents identify and describe a microbial community for performing hydrolysis and acidogenesis, describe nutrient recovery, are concerned with feedstock composition or the possibility of co-digestion or monodigestion, or employing nitrogen status control as a method for enhancing biogas production.
Ammonia removal methods other than stripping have also been used in AD. Patent application EP 2,039,775 A2 discloses a two-phase system where ammonia fermentation performed with a single, or a mix of bacterial strains, is associated with ammonia removal as gaseous ammonia through agitation of the fermented material. Ammonia is either lost to the atmosphere or recovered as such for hydrogen production, but not recovered in a form suitable for fertilizer use. In addition, the conditions used, a mildly alkaline pH of 8-8.5 and a temperature of 55-65° C. do not favor volatilization of ammonia. Patent application EP 2,614,890 A1 describes a one-phase process where ammonia removal is based on ion exchange. The method requires chemicals for regeneration of the ion exchange resin and a careful removal of solid matter from the digestate prior to application to the resin.
Patent application WO 2013038216 A1 discloses a one-phase process where a characterized microbial community is used for AD of high-protein substrates. The community is, however very different from S1, consisting by up to 50% of bacteria of the Pseudomonales order, and also methanogenic archaea that are absent from S1.
Patent application EP 2,578,558 A1 discloses nitrogen recovery from AD by stripping that is performed by recycling the produced biogas. An inorganic ammonium salt fertilizer and a mixed organic fertilizer are produced as a result. No elevated pH is used during stripping, which may cause inefficient stripping of ammonia and lead to ammonia inhibition during AD.
Patent U.S. Pat. No. 8,613,894 B2 describes methods and systems for nutrient recovery from anaerobic digester effluent with different heating and aeration systems. In this process dissolved gases such as carbon dioxide, methane and ammonia are removed after AD with the aid of elevated temperature and aeration during 12-36 hours. The described process is time-consuming and will only remove ammonia to some extent.
Patent EP 0,970,922 B1 discloses a method for removal of inhibitory substances such as ammonia from biogas reactor by membrane separation of liquid and solid components. The downside of this method is that VFA are also washed out of the reactor along with ammonia, reducing the biogas yield.
Patent EP 1,320,388 B1 discloses a process for nutrient recovery from one-phase or two-phase AD through solid-liquid separation and ammonia stripping. Also reject water is recirculated within the process. The process does not characterize the microbial community performing conversion of organic nitrogen to inorganic nitrogen, and does not utilize nitrogen status control C/N ratio for providing optimal conditions for AD.